Stylesheets are those sort of files which often seem to get sloppy quite quickly. Compared to other files in a web project these perhaps don’t get much attention after the initial build. The temptation is to just make a quick edit, check your change works, then save and get out of there. Perhaps the problem is the lack of structure compared to a fully fledged application file.
I really recommend building a WordPress theme from scratch at least once if you plan on getting in to WordPress development.